$300 seems like a good deal, better make sure there aren't any problems with it. Having ordered 3 laptops carts of 30 of those particular models, I can tell you the three main problems I've encountered that you should look for:
#1 problem: Dying or dead battery (Replaced maybe 20 bad batteries in 90 laptops.)
#2 problem: Dying or dead HDD (I've replaced about 15 out of the 90 laptops with bad HDDs.)
#3 problem: Flickering screen (only had 3 laptops out of 90 do this, but it was common enough.)
The best way to clean the laptop (especially the keyboard) is with Simple Green Industrial Towels, sometimes called Simple Green Safety Towels. It has a mild abrasive on one side of the towel, but is soft on the other. The mild abrasive is tiny, green plastic beads that don't scratch or dull the finish. It's the absolute best solution I've found for cleaning dirt and grime off a laptop. Finish with a microfiber cloth or lint-free shammy. Will look good as new.